Fabrication, measurement, and analysis of nanomechanical structures in silicon

We have developed fabrication processes capable of making very small silicon mechanical structures. The fabrication uses high-resolution electron beam lithography for the patterning of silicon-on-insulator substrates. A reactive ion etch transfers the pattern into the thin (50--250 nm) top layer of silicon. Small structures are then released in a wet hydrofluoric acid etch which attacks only the underlying oxide. A CO2 critical point drying technique is then used to prevent the destruction of devices that may occur during normal air drying. This fabrication process is capable of reproducibly fabricating suspended silicon features with lateral dimensions less than 30 nm.;We can actuate and observe motion in these structures using an optical measurement apparatus. The apparatus measures the resonant response of the structures. We have observed oscillating wires with resonant frequencies as high as 380 MHz. The measurement apparatus is capable of detecting angstrom scale resonant motion in wires with a crossection as small as 50 nm x 50 nm.;The resonant response curves provide information about the energy losses in these vibrating wires. We have measured the quality factor (Q) of these resonances and have seen a strong dependence on the size. In particular, the losses show a linear dependence on the surface to volume ratio.;We have also applied the fabrication and measurement techniques to the study of very small torsional oscillators. These structures have very interesting dynamic properties. With the application of a bias voltage, we can change the effective spring constant of the oscillator. Modulation of the spring constant at twice the resonant frequency leads to parametric amplification of the mechanical motion. This is the first observation of parametric amplification in a torsional oscillating system. A theoretical description of another type of parametric amplification utilizing two coupled oscillators is also presented.